\WP_Stager_Integration\Calendar::calculate_timestamp_month_start(int $year, int $month): int

Full Code

/**
 * Calculates the start timestamp of a given month (00:00:00 in the current site's timezone, first day of the month).
 *
 * @param int $year     The year
 * @param int $month    The month (1-12)
 *
 * @return int   Unix timestamp for month start
 */
public static function calculate_timestamp_month_start( $year, $month ) {

    // Type casting
    $year  = (int) $year;
    $month = (int) $month;

    // Get first day of month timestamp
    return (new \DateTimeImmutable( sprintf( '%d-%d-01 00:00:00', $year, $month ), wp_timezone() ))->getTimestamp();
}